Proton exchange membrane fuel cells, also known as polymer electrolyte membrane (PEM) fuel cells (PEMFC), are energy conversion devices in which hydrogen or another hydrocarbon fuel is supplied to the anode of the fuel cell and oxygen is supplied as oxidant to the cathode.
In hydrogen oxygen PEMFCs (PEMFCs in which hydrogen is supplied to the anode of the fuel cell and oxygen is supplied to the cathode of fuel cell) comprise a membrane electrode assembly (MEA) consisting of a proton conducting polymer membrane functioning as the electrolyte which separates an anode from a cathode. Hydrogen is introduced at the anode where it comes into contact with a catalyst, causing dissociation of the hydrogen into constituent protons and electrons. This oxidation half-cell reaction or Hydrogen Oxidation Reaction (HOR) is represented by Eq. 1 below.H2→2H++2e−  (Eq.1)
Once formed, the protons pass from the anode through the polymer electrolyte membrane to the cathode. The transport process of the protons across the polymer electrolyte membrane is facilitated by interactions of the protons with one another as well as with the water molecules (which is the by-product of the electrochemical reaction in the fuel cell) in the MEA. However, the electrons cannot pass through the polymer electrolyte membrane and instead traverse an external circuit that connects the anode to the cathode, thus generating the current output of the fuel cell.
Meanwhile, a stream of oxygen is delivered to the cathode of the MEA. At the cathode, oxygen molecules react with the protons permeating through the polymer electrolyte membrane and the electrons arriving through the external circuit to form water molecules. This reduction half-cell reaction or oxygen reduction reaction (ORR) is represented by Eq. 2 below.½O2+2H++2e−→H2O  (Eq. 2)The overall reaction in the hydrogen oxygen PEMFC is represented in Eq. 3 below.H2+½O2→H2O  (Eq. 3)
PEMFCs require active and stable catalysts at both at the anode and at the cathode where the fuel oxidation and oxygen reduction reactions occur, respectively. The catalysts are needed to speed up the rates of the electrochemical reactions, which are particularly sluggish on the cathodic side of the fuel cell device. The rate of the oxygen reduction reaction is approximately 102-103 times lower than the rate of the anodic reaction. Currently, PEMFCs use platinum catalysts such as carbon-supported platinum and platinum-transition metal alloy catalysts. However, improved catalysts, particularly improved catalysts for the ORR, that are less expensive, exhibit improved stability, and/or exhibit sustained or improved activity are needed to develop commercially viable fuel cells.